Exception Handling in async in Kotlin

Introduction

Exception Handling in async is a fundamental concept every Kotlin developer should understand. async starts parallel work and await retrieves its result. Together they compose concurrent operations without manual thread management.

Exceptions in async are stored in Deferred until await. In this tutorial you will learn the syntax, walk through a complete example program, study the sample output, and review best practices so you can apply the concept confidently in your own projects.

Definition

• Exceptions in async are stored in Deferred until await.
• Use try-catch around await to handle failures.
• SupervisorJob can isolate child failures.

Syntax

try { deferred.await() } catch (e: Exception) { }

Exception Handling in async in Kotlin Example Program in Kotlin

import kotlinx.coroutines.*

fun main(args: Array<String>) = runBlocking {
    val deferred = async {
        throw IllegalStateException("Failed")
    }
    try {
        deferred.await()
    } catch (e: Exception) {
        println("Error: ${e.message}")
    }
}

Sample Output

Error: Failed

When to use

Use async/await when you have independent tasks that can run in parallel and you need to combine their results.

How it works

1. The program starts with a main function — the entry point that runs when you execute the file.

2. val deferred = async { assigns or updates a value used later in the program.

3. The println("Error: ${e.message}") statement writes a line to the console — this produces part of the sample output below.

4. Exceptions in async are stored in Deferred until await.

5. Run the program in IntelliJ IDEA, Android Studio, or with the Kotlin command-line compiler (kotlinc / kotlin). Compare your console output with the sample output shown below.

Best Practices

Common Mistakes

Key Points

• Exceptions in async are stored in Deferred until await.
• Use try-catch around await to handle failures.
• SupervisorJob can isolate child failures.
• Test the example locally and verify the output matches the sample.
• Experiment by changing input values to see how behaviour changes.

Notes

• Add the kotlinx-coroutines-core dependency when running coroutine examples outside Android or IntelliJ.
• Semicolons at the end of statements are optional in Kotlin.